1. Field of the Invention
The present invention relates to a D channel packet communication system and a method of D channel packet communication, and more particularly, to a D channel packet communication system and a method of D channel packet communication for an ISDN (Integrated Services Digital Network) if a private branch exchange in which a D channel packet is discriminated by a line circuit, and connection control information contained in the D channel packet is converted and transferred to an ISDN network through a permanent connection path established in a time division switch without passing through a call processor.
2. Description of Related Art
A private branch exchange (hereafter referred to as PBX) is used in corporations, schools, hotels and some other places for providing an internal private communications network. The PBX accommodates its own terminals of telephones, data communication terminals and other kind of terminals for internal communications and communications to a public communication network. Therefore, the PBX performs switching operations for calls to and from those terminals, and establishes communication paths not only within the private communications network, but also to public communications networks.
On the other hand, a PBS may also be regarded as a subscriber terminal of a public switching system in a public communications network. That is, although the PBX itself accommodates an unspecified number of terminals, the PBS is treated as a subscriber terminal in control operations of the public switching system as far as the public communications network is concerned. Therefore, where the public communications network provides the ISDN, then a TEI (Terminal Equipment Identifier) and some other service parameters to be allocated for subscriber terminals of a public ISDN switching system are also allocated to the PBX. Those parameters are allocated to each subscriber line which interconnects a central office trunk (hereafter referred to as trunk) of the PBX and a line circuit of the public ISDN switching system depending on service conditions of the PBX as a subscriber terminal, but are not allocated to each terminal accommodated in the PBX.
FIG. 1 is a block diagram illustrating a configuration of a conventional D channel packet communication system in a PBX performing D channel packet communication without the D channel packet passing through a call processor (Japanese Laid Open Patent Application No. HEI 2-226844).
In this figure, the PBX 2 is provided with a line circuit 4 accommodating a packet terminal 1 as an internal terminal, a trunk circuit 5 connected to a public-ISDN switching system (not shown) in a public communications network 3 (hereafter referred to as network), a time division switch 7 for establishing a communication path between the line circuit 4 and the trunk circuit 5, and a call processor 6 for controlling the overall operations of the PBX 2. A switching control unit 41 is provided in the line circuit 4 and a switching control unit 51 is provided in the trunk circuit 5. Those switching control units 41 and 51 perform connecting switching for the flow of received D channel information to either side of the division switch 7 or the call processor 6.
FIG. 2 is a diagram showing the sequence of D channel packet communication performed by the conventional D channel packet communication system in the PBX 2 of FIG. 1.
When the packet terminal 1 at the sending side requests D channel packet communication, "link set up" information of D channel information is transmitted from the packet terminal 1 to the network 3 based on subscriber protocols of packet communications defined by CCITT (International Telegraph and Telephone Consultative Committee, at present ITU-T: International Telecommunication Union Telecommunication Standardization Sector) recommendations Q.931.
The "link set up" information is the information for requesting the network 3 to establish a communication path, and this information includes a SAPI (Service Access Point Identifier) of the layer 2 protocol information for indicating the mode of data communication to be followed. As the D channel packet communication based on CCITT recommendations X.25 protocol is followed in this case, a value of "16" is set in the SAPI, i.e., the "link set up" information with SAPI-15 means requesting a communication path for the communication of D channel packet based on CCITT recommendation X.25 protocol.
The switching control unit 41 of the line circuit 4 is normally connected to the call processor side, so the "link set up" information is transferred to the call processor 6 from the line circuit 4 which has received the "link set up" information and terminated the layer 2 protocol. The call processor 6 analyzes the "link set up" information and recognizes that the mode of communication requested is D channel packet communication by recognizing the value of SAPI which has been set to "16". Then, the "link set up" information is transferred from the call processor 6 to the network 3 through the trunk circuit 5.
When the network 3 responds to the "link set up" information by transmitting the "link set up confirmation" information, since the switching control unit 51 of the trunk circuit 5 is normally connected to the call processor side, the "link set up confirmation" information from the network 3 is transferred to the call processor 6 from the trunk circuit 5. Then, the call processor 6 identifies the information as a response to the "link set up" information and transfers the information to the packet terminal 1 through the switching control unit 41 of the line circuit 4.
Once the "link set up confirmation" information has been acknowledged by the circuit 4, the trunk circuit 5 and the call process 6, each of the switching control units 41 and 51 switches its connection path from the call processor side to the time division switch side, and the call processor 6 establishes a communication path between the line circuit 4 and the trunk circuit 5 in the time division switch 7 using predetermined connection information.
Throughout the procedures described above, the packet terminal 1 which has received the "link set up confirmation" information is connected to the network 3 through the line circuit, 4 the time division switch 7 and the trunk circuit 5.
Next, the packet terminal 1 transfers the "X.25 call request" information to a packet terminal at the receiving side (not shown) in the network 3 for requesting commencement of a packet communication with the X.25 protocol. When a link set up at the packet terminal of the receiving side has been completed, the "X. 25 call termination completion" information in response to the "X.25 call request" information is sent back from the packet terminal of the receiving side to the packet terminal 1 at the sending side, whereafter D channel packet communication commences.
At the end of communication, the "X.25 disconnect request" information is transferred from the packet terminal 1. When the line circuit 4 and the trunk circuit 5 recognize that the "X.25 disconnect request" information has passed therethrough, each of the switch control units 41 and 51 switches its connection path from the time division switch side to the call processor side. Therefore, the "X.25 disconnect confirmation" signal in response to the "X.25 disconnect request" signal sent from the network 3, is received by the call processor 6. The call processor 6, having received the "X.25 disconnect confirmation" signal, transfers this information to the packet terminal 1 through the switch control unit 41 of the line circuit 4. Thereafter, the call processor 6 disconnects the communication path between the line circuit 4 and the trunk circuit 5 in the time division switch 7 when the "link disconnection" information is transmitted from the packet terminal 1 based on the Q.(31 protocol.
The conventional D channel packet communication system and the method of D channel packet communication in the PBX as described above, however, have the following problems:
(1) The call processor must bear the processing load each time D channel packet communication is performed, because the call processor is required for processing the D channel information based on the Q.931 protocol;
(2) The efficiency of the time division switch is degraded because the communication path between the line circuit and the trunk circuit is established for each D channel packet communication and connection paths on the time division switch are occupied by particular packet terminals such as the PVC (Permanent Virtual Circuit) type terminal or a terminal which does not release a communication path even at the end of communication; and (3) Only a limited range of services are available for packet terminals which meet the service conditions allocated to the PBX as a subscriber terminal of the public ISDN switching system in the network, because the D channel information transmitted from the packet terminal is only passed through to the network without any conversions.